A new letter from exiled Soviet dissident Andrei Sakharov seems to support Reagan's defense buildup.
Written in April and scheduled to have been read at a scientific conference in Warsaw this week, the message says ''substantial increases'' in Soviet armed forces during the 1970s have upset the East-West military ''equilibrium'' and endangered the chances for world peace.
It is necessary for the West to respond, the Nobel Prize laureate writes, ''to restore strategic parity in the field of conventional weapons.'' At the same, he adds, ''disarmament negotiations must be held constantly and persistently despite all the difficulties entailed.''
The letter appeals to the West to come to the aid of a long list of imprisoned Soviet intellectuals and notes that ''persecution of dissidents (in the USSR) has increased.''